1. Field of the Invention
This invention relates to a method of testing semiconductor wafers to locate problems resulting from the fabrication procedure.
2. Brief Description of the Prior Art
The fabrication procedure for fabrication of semiconductor devices generally requires several hundred steps. It is necessary that each step be repeatable and that all wafers fabricated during any step be subjected to the same conditions to the greatest extent possible in order to insure uniformity of device parameters within some predetermined window from wafer to wafer as well as to insure uniformity of parameters across a wafer.
In order to maximize such uniformity, a common technique for determining the process step responsible for a specific type of wafer to wafer parameter variation has been to process a lot of wafers through different pieces of equipment with the wafers being randomized in the loading cassette in order to randomize the order in which they are processed through various manufacturing equipments. Alternatively, wafers may be processed in a specific numeric order in specific equipments, different orders being associated with different pieces of equipment. The average parametric value or spatial variation of a parameter over the wafer is then correlated with the wafer processing sequence through each equipment in order to identify specific equipment for which the parametric variation sequence correlates with the processing order sequence. As part of this procedure, a unique readable number or bar code feature is provided on each wafer, the wafers are physically randomly rearranged from process step to process step or in some order less than every step, and the location of each wafer in the lot is read periodically by reading the readable number thereon and the location. A parameter or parameters are then checked afterwards for each wafer and correlated to the wafer order at each process step where wafers were randomized to determine any parameter differences from wafer process order and to determine how processing may have caused these parameter differences. While this method of randomizing the order of the wafers through various pieces of equipment is effective in isolating which piece of equipment is responsible for trends in the parameters from wafer-to-wafer within a lot, it does not help in determining which piece of equipment causes trends in parameters across a wafer such as a top of wafer to bottom of wafer variation or a left side of wafer to right side of wafer variation. To isolate the equipment causing this type of variation it is desirable to supplement the randomization of wafer processing order with a method to orient wafers at different angles before certain process steps so that it would be possible to explore variations in across-wafer parametrics as a function of different process steps. This has not been currently done in any systematic manner to date, such as by using specific or randomized rotational variations at designated process steps and then tracking the associated parametric variation on wafers. However, at present, there is no known procedure for automatically performing the function of wafer rotation in addition to the rearrangement of the wafer processing order with subsequent parametric tracking.